Focus accuracy on A700 with 50mm f1.4 and 100mm Macro

[Jonathan K]

I believe the Canon/Nikon high-end cameras have some option to adjust every single lens....
Don't ask me more, I was told by somebody who was told by somebody

Jonathan

[Glyn R]

The dphoto expert comments on live view only apply cameras where the Live view image is taken directly from the main sensor. The Sony system is only really a view of the viewfinder image so focus errors will creep in. This was apparent on my E-330 which had both systems, manual focusing using live view was only accurate with the main sensor viewer.

[lock]

The firmware already contains an adjustment table. It only covers one lens, the 50mm f1.7, and it only corrects for an aperture of f7.1 and a working distance of 1.93 metres. One reason the fast lenses like the 50mm f1.7 may misfocus at full aperture is because the adjustment table is set to assume f7.1 as the working aperture, and any focus shift occurring will be compensated for at that aperture - but wide open will not be perfect, as the focus shift will be assumed to be present when it's not.

Agreed - they need a far more complex adjustment table, covering the aperture set, the focal length and the focus distance. Maybe the Alpha 900 will have it.

David

David and Jonathan, thank you for bringing up this subject. What you are referring to, the so called adjustment table, probable affects every lens. Particularly the fast ones. Apart from the fact that i'm often fooled by the wide F/2.8 sensor, my Tamron 17-50 also seems to FF consistently . At about 1.5 meters I think it amounts up to 6 or 7 cm. But at 10 meters it quickly creeps up to at 1.5 meters !
I've done quite a lot of testing, ending up with throwing away the test charts because they are way too sensitive to the setup of your gear. But simple things were just as much capable of showing the FF and there is even a hint that the wide AF sensor has a slightly different FF than the upper and lower sensors.

Now the question I would like to ask. Does every lens that communicates with the body provide info to specify how the body should actually adjust, thereby overruling the default adjustments based on the single the 50mm f1.7 you mentioned ?

cheers,

lock

[David Kilpatrick]

Easy answer - no. They do not provide any AF offset information, just their focal length, aperture, focus distance current in use, etc.

David

[01af]

Wonder why they don't add AF micro adjustment to the A700 via another firmware update---now after the A900 has it. It should be doable in firmware, shouldn't it? If it was able to store fine adjustments for, say, five or six lenses only I'd be happy. With that feature added I would buy an A700 right away.

-- Olaf

[Dr. Harout]

Wonder why they don't add AF micro adjustment to the A700 via another firmware update---now after the A900 has it. It should be doable in firmware, shouldn't it? If it was able to store fine adjustments for, say, five or six lenses only I'd be happy. With that feature added I would buy an A700 right away.

-- Olaf

I was just thinking the same thing and hoping they will do it.

[lock]

Easy answer - no. They do not provide any AF offset information, just their focal length, aperture, focus distance current in use, etc.

David

Well, I've read that for near distance focussing lens systems use correction data (United States Patent 4747677, Olympus). This type of data is provided by lenses, e.g. my Sigma 17-70 on a Pentax:
Near Distance Abberation Correction Data (LC4): 84 At least, it's in the EXIF.

There is more, but I'm not sure it's coming from the lens only. Things like full aperture metering error compensation data, which obviously refers to the metering system.

OT, though related, there are reports about V4 firmware ruining the AF.

[David Kilpatrick]

Take a look at my reports from the conference and the dinner floor show - all shot using AF and v4, in some very low light and varied conditions. Absolutely spot-on consistent AF throughout. v4 certainly has not ruined the AF on my A700!

As for the lens EXIF data, I do not see how a double-digit code can possible express anything useful to do with Near-Distance Focusing Aberration Correction. While such a code could adjust for full aperture metering (T-stop versus f-stop etc), any aberration correction requires a complex mapping function. One camera I know of includes double-digit code recognition to call up such a function - the Kodak DCS SLR Pro/n or Pro/c. But it uses lens identity to do so, and relies on a downloaded list of the corrections for shading and CA. It does not attempt to correct any kind of distortion.

The Hasselblad H*D series with the smaller (31 not 39 megapixel etc) sensor include distortion and vignetting/CA correction for the 28mm f4 wide angle but this is a unique special case - the lens is unusable without this correction.

Your Near Distance focusing correction could be shifting focus because at close distances there is an aperture-related focus shift. But again, a single correction is insufficient. A range of near distances would be needed, and a range of apertures - two adjustment function curves with an interactive calculation. If the lens is reporting a different figure (84 etc) depending on the f-stop set and the focused distance, then it might contain such a table of its own, and give a tailored offset to the camera for each shot. Can you check that?

David

[lock]

David, this is not a thorough check cause i do not have a large number of my files with me.
But is far as I can at least two things of influence on Near Distance Abberation Correction Data (LC4) and Light Color Aberration Correction Data (LC5): aperture and the amount of mm of the zoom lens. Of course this is fixed with a prime...There is a hint that the type of light (daylight, artificial light indoors) makes a difference but I could be wrong. I simply do not have enough samples to verify.
There is at least one fairly steady type of data: mv' nv' Data (LC10): full aperture metering error compensation data. On the 17-70 it's 128, but from this lens none of the pictures are wide angle. The 50-200 reports two values: 112 (only at 50 mm) or 121.

Is this what you mean ?

[David Kilpatrick]

Yes, if the reported figure changes with settings on the lens, it may have some function. But not on all cameras, and it may not affect the focusing, just metering on embedded info for use during raw conversion. Without more details being revea;ed of some new industry standard for communicating correction-related data, I can not guess how a Pentax and a Sony might share any common ability to use such information from third party lenses.

David